Ion Beam Analysis
Ion Beam Analysis (IBA) utilizes high-energy ion beams to probe elemental composition non-destructively as a function of depth to several microns with a typical depth resolution of 100-200 angstroms. It is a fast, nondestructive and standardless technique to quantify the absolute atomic ratios in compounds or mixtures, insensitive to their chemical environments. It can also determine the film thickness (or density) as well as the structural disorders in single crystalline targets. Energy distribution of backscattering ions quantifies the depth distribution for a given element. Distinctive characteristic X-rays emitted from the different target elements upon the beam bombardment ensure the accurate identification of similar mass elements. Gamma rays emitted from the beam-induced nuclear reactions provide an excellent sensitivity (~ppm) and/or depth resolution (~50 A) for certain light isotopes such as 1H, 15N, and 19F.
IBA is a broad term that involves several specific techniques, mainly:
- Rutherford backscattering spectrometry (RBS)
- Forward recoil spectrometry (FReS) or elastic recoil detection analysis (ERDA)
- Nuclear reaction analysis (NRA)
- Particle induced X-ray emission (PIXE) analysis
- Ion channeling analysis
MAS 1700 pelletron tandem ion accelerator (5SDH) equipped with charge exchange RF plasma source by National Electrostatics Corporation (NEC).
Analytical endstation (RBS 400) and control software (HYPRA) by Charles Evans & Associates:
Graphic-interface computer control of data acquisition and ion beam characteristics:
- Fixed ion detector at 165° for Rutherford backscattering spectrometry (RBS).
- Movable ion detector (90°- 150°) for grazing-angle scattering (enhanced surface sensitivity).
- Fixed ion detector at 30° for forward-recoil spectrometry (FReS) of light target ions such as H and D.
- Scintillation NaI(Tl) gamma-ray detector for Nuclear Reaction Analysis (NRA) of light elements.
- Retractable Si(Li) X-ray detector for particle induced X-ray emission (PIXE) analysis of trace elements.
- Sample goniometer controlled translational and rotational movement.
- Computer control of ion beam parameters (mass, energy, charge, current and focusing).
- Automated collection of data on multiple samples.
- Sample positioning/tilting (automatic orientation for axial or planar channeling).
- Simultaneous collection of RBS, FReS, NRA, and PIXE spectra.
- Multitasking software allows one to manipulate previous results while continuing to acquire data.
- Cryogenic sample stage, liquid-nitrogen cooled (to reduce beam damage on organic samples).
- Different sizes of apertures available to control beam size on target from 0.2 mm to 10 mm.
- Auxiliary analysis workstations networked.
- HYPRA, RUMP, and GISA-3 software available for IBA data analysis.
- Color printing from both control and analysis stations, batch data-file format conversion.
Rutherford backscattering spectrometry (RBS):
- Nondestructive and multielemental analysis technique
- Elemental composition (stoichiometry) without a standard (1-5% accuracy).
- Elemental depth profiles with a depth resolution of 5 - 50 nanometers and a maximum depth of 2 - 20 microns.
- Surface impurities and impurity distribution in depth (sensitivity up to sub-ppm range).
- Elemental areal density and thus thickness (or density) of thin films if the film density (or thickness) is known.
- Diffusion depth profiles between interfaces up to a few microns below the surface.
- Channeling-RBS is used to determine lattice location of impurities and defect distribution depth profile in single crystalline samples
Forward recoil spectrometry (FReS):
- Nondestructively and simultaneously determines hydrogen isotopes (H and D) and their depth profiles in polymers and other solids with a sensitivity of 0.01 at.%, a depth resolution of 30-80 nm, and a maximum depth of 1 micron.
- Measurements of other light elements (Z<9) are also possible if heavy ions like Cl or Au are used.
Nuclear reaction analysis (NRA):
- Nondestructively measures light elemental depth profiles (Z<9) with a superb sensitivity of a few ppm, a good depth resolution of a few nanometers, and a maximum depth of a few microns. Elements like H, D, Li, B, C, O, and F can be analyzed.
- Unlike ion scattering techniques, NRA is an isotopically sensitive technique with an excellent mass resolution and has no mass-depth ambiguity of RBS and FReS in data interpretation.
- Channeling-NRA can be used to determine lattice location of impurities and defect distribution depth profile in single crystalline samples
Particle induced X-ray emission (PIXE) analysis:
- Nondestructive and multielemental analysis of trace elements with an excellent detection limit of up to 20 ppb.
- Used together with RBS for accurate mass identification of medium to heavy elements with similar masses.
- Elemental composition of magnetic films in which RBS does not have an enough mass resolution to resolve Mn-Fe-Co-Ni elements.
- Channeling-PIXE can be used to determine lattice location of impurities in single crystalline samples
Ion channeling analysis
- Assess crystallinity of MBE-grown thin films such as type of defect structures, impurity location, type of atomic site, lattice strain and alignment in epitaxial growth
- Enhance surface sensitivity of light elements on heavier single crystal substrate
- Channeling-RBS, Channeling-NRA, and Channeling-PIXE are available for different applications
Ion beam specifications:
- Accelerator terminal voltage tunable from 80 kV to 1.7 MV (source injection voltage is up to 30 kV).
- H+, He+ and He++ beams in standard configuration with maximum energies of 3.4, 3.4 and 5.1 MeV, respectively.
- Beam spot size from 0.2 mm to 1 mm.
- Beam current on target up to a few tens to hundreds nA depending on ion species and energies.
- 3He, 15N and 16O beams are also available for nuclear reaction analysis (NRA) or elastic recoil detection analysis (ERDA).
Particle detector specifications:
- Ortec Ultra ion detectors: energy resolution of 12 keV
- Kevex Retractable Si(Li) X-ray detector with 5 m
m Be-window: energy resolution of 145 eV.
- Canberra 2"x2" NaI(Tl) gamma detector: energy resolution of 6.5%.
- Sample lateral movements: ±
25mm with a minimum step size of 0.001mm.
- Tilting movements: ± 90° along vertical axis and ± 20 °
along horizontal axis with a minimum step of 0.01°.
- Typical sample size: 5x5 mm2 for RBS/NRA/PIXE/Channeling and 5x15 mm2 for FReS/ERDA.
- Minimum size: 0.5x0.5 mm2 and Maximum size: 50x50 mm2
- Sample thickness is no more than 5 mm in standard configuration, but special arrangement may be done for thicker ones.
- Sample has to be vacuum compatible solid with relative smooth surface.
The above compares data taken at random sample orientation (unchanneled)
and under axial channeling conditions. The colored arrows denote the energies
of backscattered He nuclei from different elements at two depth locations.
A 3.5 MeV beam energy enables certain nuclear reactions, thereby increasing
the sensitivity to N and C.
Last update: October 2003